STAR BURST Dendrimers (D) and Ultra Small Iron Oxide Particles (USPIO) were developed for possible cellular tags in molecular imaging. A series of high-generation (G) dendrimers (G = 5, 7, 9, 10) were conjugated to DOTA, and gadolinium (III) ion was added to the 1/T1 and 1/T2 NMR dispersion profile for generations 5 through 10 dendrimer DOTA complexes as reported this year by Bryant et al, JMRI 1999;9:348-352. These results showed an increase in proton relaxation enhancement effect (PRE) from G 5 through G 7 and then a leveling off at generations 9 and 10. Biodistribution studies in rodents using radiolabeled gadolinium 153 chelated to the DOTA-dendrimer complex revealed dose-dependent effects for blood half life and tissue distribution. These results suggest that at lower doses of G5 G10 gadolinium DOTA-dendrimer complex, there is preferential uptake by the reticuloendothelial cell system (primarily in liver, spleen, bone marrow), which can be saturated at higher doses of the agents. There is no evidence of enzymatic breakdown of the gadolinium-DOTA dendrimers based on dynamic NMR relaxation time experiments performed on whole blood in vitro. USPIO particles, as specific targeting agents to cerebral endothelial markers for trans-ferring receptor (Tfr), were used to tag the rat oligodendrocyte precursor cell line CG-4 with MION- 46L. CG-4 MION-labeled cells had viability greater than 95 percent and at 7 days it was greater than 80 percent. CG-4 MION-labeled progenitor cells were implanted into the spinal cords of myelin-deficient rats. In vitro magnetic resonance microscopy at 4.7 Tesla revealed migration of iron oxide-labeled cells up to 8 to 10 mm over 14 days post- implantation from the surgical site. On histopathology, axons were shown to be myeli-nated and the extent correlated to the magnetic resonance (MR) findings. Further work is planned to determine the duration that the MION labels will remain visible using in vivo MR imaging techniques.